The recent increasing interest in the development of alternative fuel sources has lead to a number of new synthetic challenges. At the forefront of many research efforts is the advancement of hydrogen fuel cell technology. Fuel cells offer significant economical, environmental and operational benefits over conventional fuels. Fuel cells function by converting the chemical energy stored in molecular hydrogen directly to electrical energy.
Direct methanol and H2/O2 proton exchange membrane fuel cells are promising power generators for terrestrial and space applications where high energy efficiencies and high power densities are required. An important component of these devices is the proton conducting membrane. For a cation exchange membrane to be used in such fuel cells, a number of requirements are to be met, including high ionic (protonic) conductivity, dimensional stability (low/moderate swelling), low electro-osmotic water flow, mechanical strength and chemical stability over a wide temperature range, a high resistance to oxidation, reduction, and hydrolysis, and low hydrocarbon fuel cross-over rates (e.g., low methanol cross-over for direct methanol fuel cells). To date, those membranes reported in the open literature that conduct ions (protons) at moderate temperatures also possess high methanol permeability and those membranes that do not transport methanol have a low proton conductivity.
Perfluorosulfonic acid (PFSA)-based membranes (e.g., Nafion) have been found to possess most of the properties needed for good performance in fuel cells. In conventional near-ambient temperature fuel cells, the polymer electrolytes, such as Nafion, require hydration for suitable operation. The proton conductivity of the membrane is substantially reduced above 80° C. due to a loss of hydration of the membrane. The proton conductivity at elevated temperatures (>100° C.) of a conventional Nafion membrane can be improved by running the fuel cell under pressure, however the complexity of the system increases and the power density is reduced in the overall fuel cell system. These high temperature limitations are particularly problematic because fuel cells are known to have a higher performance at temperatures of 150° C. and above.
Prior art fuel cell membranes containing imidazoles, as seen in FIG. 1, are currently under investigation. The use of imidazoles in place of water in membranes produces proton conductivities between 150° C. and 250° C. that are comparable to that of hydrated polymers. For hydrated polymers to be useful at such temperatures, it is necessary to enclose the hydrated polymers in closed electrochemical cells in order to avoid the loss of water. Similarly heterocycles, such as imidazoles, may also escape from open fuel cells. Imidazoles that have been immobilized by being bound to oligomers, as seen in FIG. 2, have been shown to have proton conductivities of 5×10−3 S cm−1 at 120° C. in substantially water-free materials. Also under investigation is the immobilization of heterocycles on polymer supports, as seen in FIG. 3, that are capable of providing high mobility for proton conductivity. It has been reported that conductivities of the order of 10−3 S cm−1 at 200° C. using polymer bound imidazoles and suggest this corresponds to a high mobility of protonic charge carries of 10−5 cm2 s−1. In these systems higher conductivity is directly correlated with the ratio of the imidazole groups to inert polymer support.
Notwithstanding the state of the art as described herein, there is a need for further improvements in preparing N-heterocyclic functionalized polymers, such as triazole functionalized phosphazene trimers or triazole functionalized polyphosphazenes, with greater ratios of proton conducting sites to polymer support and polymer supports that have high thermal stability are needed. Materials possessing these properties may provide an opportunity to develop more efficient non-hydrated polymer electrolyte membrane (PEM) fuel cells.